CENTRALIZATION AND CEPHALIZATION 147 



arachnids the process of cephalization appears in one 

 form or another. To what extent these processes occur 

 in the echinoderms is uncertain, but the holothurians 

 probably represent a somewhat more advanced cephali- 

 zation than the other groups. And finally the progress 

 of nervous cephalization from the lower to the higher 

 vertebrates is perhaps the most significant feature of 

 vertebrate evolution. The evolutionary progress of 

 nervous cephalization is accompanied by various other 

 changes in form, proportion and relations of parts, par- 

 ticularly in the segmented animals, in which the fusion 

 of segments at the anterior end of the body, the decrease 

 in total number of segments, and the increasingly 

 intimate physiological relations of the segments in 

 general are characteristic features. 



Concerning the physiological basis of these changes 

 we know as yet practically nothing, but it is evident 

 that the changes represent modifications of the axiate 

 pattern with respect to the longitudinal axis, and if the 

 gradient is the physiological basis of axiate pattern, 

 these evolutionary changes must be interpreted physio- 

 logically in terms of gradients. Undoubtedly the form 

 or curve of a physiological gradient, its slope or steep- 

 ness, is different in different kinds of protoplasm, and 

 undoubtedly also the capacity of protoplasm to develop 

 a high conductivity has increased in the course of evo- 

 lution. The experimental data at hand do not afford a 

 basis for exact quantitative measurement or comparison 

 of the gradient curves in different organisms, but they 

 do indicate that the gradients undergo changes in the 

 course of evolution as well as in ontogeny. The evolu- 

 tionary change means merely that as the constitution 



